Radiation beam alignment for medical linear accelerators

Abstract

Radiation beam alignment for a LINAC including (1) for each beam alignment parameter value of a set: (a) with a beam alignment parameter of a LINAC set to the beam alignment parameter value, using a gantry to generate a radiation beam; (b) using an imaging device to acquire a radiation transmission image indicative of a radiation field of the radiation beam after passing by a radiation opaque marker; (c) determining a location of a beam axis of the radiation beam and a center of a shadow of the marker based on the radiation transmission image; and (d) determining a target-to-beam-axis distance between the location of the beam axis and the center of the shadow of the radiation opaque marker; and (2) determining an optimum beam alignment parameter value based on the beam alignment parameter values and the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values.

What is claimed is: 
     
       1. A method comprising:
 for each beam alignment parameter value of a set of beam alignment parameter values:
 with a beam alignment parameter of a linear accelerator (LINAC) set to the beam alignment parameter value, using a gantry of the LINAC to generate a radiation beam; 
 using an imaging device of the LINAC to acquire a radiation transmission image indicative of a radiation field of the radiation beam after passing by a radiation opaque marker; 
 determining a location of a beam axis of the radiation beam and a center of a shadow of the radiation opaque marker in the radiation field of the radiation beam based on the radiation transmission image; and 
 determining a target-to-beam-axis distance between the location of the beam axis of the radiation beam and the center of the shadow of the radiation opaque marker in the radiation field of the radiation beam; and 
 
 determining an optimum beam alignment parameter value based on the beam alignment parameter values of the set of beam alignment parameter values and the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values. 
 
     
     
       2. The method of  claim 1 , further comprising, for each beam alignment parameter value of the set of beam alignment parameter values, setting the beam alignment parameter of the LINAC to the beam alignment parameter value. 
     
     
       3. The method of  claim 1 , wherein the beam alignment parameter of the LINAC is an amount of current applied to a bending magnet of the LINAC. 
     
     
       4. The method of  claim 1 , wherein the optimum beam alignment parameter value is determined such that a target-to-beam-axis distance between a location of a beam axis of a radiation beam generated by the LINAC with the beam alignment parameter set to the optimum beam alignment parameter value and a center of a shadow of the radiation opaque marker in the radiation field of the radiation beam generated by the LINAC with the beam alignment parameter set to the optimum beam alignment parameter value would be zero. 
     
     
       5. The method of  claim 1 , wherein determining the optimum beam alignment parameter value comprises:
 determining a function that models a dependence of the target-to-beam-axis distance on the beam alignment parameter based on the beam alignment parameter values of the set of beam alignment parameter values and the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values; and 
 using the function to determine a beam alignment parameter value at which a target-to-beam-axis distance would be zero. 
 
     
     
       6. The method of  claim 5 , wherein the function is a first degree polynomial function. 
     
     
       7. The method of  claim 5 , wherein determining the function comprises performing a linear least square fit for the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values. 
     
     
       8. The method of  claim 1 , further comprising:
 setting the beam alignment parameter of the LINAC to the optimum beam alignment parameter value; and 
 with the beam alignment parameter of the LINAC set to the optimum beam alignment parameter value, using the LINAC to generate a radiation beam. 
 
     
     
       9. The method of  claim 1 , wherein determining the location of the beam axis of the radiation beam comprises determining a center of the radiation field of the radiation beam based on the radiation transmission image, and the determined location of the beam axis of the radiation beam is the determined center of the radiation field of the radiation beam. 
     
     
       10. The method of  claim 1 , wherein determining the location of the beam axis of the radiation beam comprises:
 determining a first center of the radiation field of the radiation beam based on the radiation transmission image; 
 rotating a collimator of the LINAC by 180 degrees; 
 using the imaging device of the LINAC to acquire a second radiation transmission image indicative of the radiation field of the radiation beam with the collimator rotated by 180 degrees; 
 determining a second center of the radiation field of the radiation beam based on the second radiation transmission image; and 
 averaging the first and second centers, wherein the determined location of the beam axis of the radiation beam is the average of the first and second centers. 
 
     
     
       11. The method of  claim 1 , wherein the radiation opaque marker is positioned in the radiation field of the radiation beam at an axis of rotation of a collimator of the gantry of the LINAC for one or more gantry angles. 
     
     
       12. An apparatus configured to:
 for each beam alignment parameter value of a set of beam alignment parameter values:
 with a beam alignment parameter of the a linear accelerator (LINAC) set to the beam alignment parameter value, use a gantry of the LINAC to generate a radiation beam; 
 use an imaging device of the LINAC to acquire a radiation transmission image indicative of a radiation field of the radiation beam after passing by a radiation opaque marker; 
 determine a location of the beam axis of the radiation beam and a center of a shadow of the radiation opaque marker in the radiation field of the radiation beam based on the radiation transmission image; and 
 determine a target-to-beam-axis distance between the location of the beam axis of the radiation beam and the center of the shadow of the radiation opaque marker in the radiation field of the radiation beam; and 
 
 determine an optimum beam alignment parameter value based on the beam alignment parameter values of the set of beam alignment parameter values and the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values. 
 
     
     
       13. The apparatus of  claim 12 , wherein the apparatus is further configured to cause the LINAC to, for each beam alignment parameter value of the set of beam alignment parameter values, set the beam alignment parameter of the LINAC to the beam alignment parameter value. 
     
     
       14. The apparatus of  claim 12 , wherein the gantry comprises a bending magnet, and the beam alignment parameter of the LINAC is an amount of current applied to the bending magnet. 
     
     
       15. The apparatus of  claim 12 , wherein the optimum beam alignment parameter value is determined such that a target-to-beam-axis distance between a location of a beam axis of a radiation beam generated by the LINAC with the beam alignment parameter set to the optimum beam alignment parameter value and a center of a shadow of the radiation opaque marker in the radiation field of the radiation beam generated by the LINAC with the beam alignment parameter set to the optimum beam alignment parameter value would be zero. 
     
     
       16. The apparatus of  claim 12 , wherein the apparatus is configured to, in determining the optimum beam alignment parameter value:
 determine a function that models a dependence of the target-to-beam-axis distance on the beam alignment parameter based on the beam alignment parameter values of the set of beam alignment parameter values and the target-to-beam-axis distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values; and 
 use the function to determine a beam alignment parameter value at which a target-to-beam-axis distance would be zero. 
 
     
     
       17. The apparatus of  claim 16 , wherein the apparatus is configured to, in determining the function, perform a linear least square fit for the target-to-axis-beam distances determined with the LINAC set to the beam alignment parameter values of the set of beam alignment parameter values. 
     
     
       18. The apparatus of  claim 12 , wherein the apparatus is further configured to:
 set the beam alignment parameter of the LINAC to the optimum beam alignment parameter value; and 
 with the beam alignment parameter of the LINAC set to the optimum beam alignment parameter value, use the LINAC to generate a radiation beam. 
 
     
     
       19. The apparatus of  claim 12 , wherein the gantry comprises a collimator and a bending magnet. 
     
     
       20. The apparatus of  claim 12 , wherein the radiation opaque marker is positioned in the radiation field of the radiation beam at an axis of rotation of a collimator of the gantry of the LINAC for one or more gantry angles.